Hello,

In your creation filter, you also need to specify the rectangle (Rect) for the new UIElement you are adding. Here is a sample creation filter I quickly put together to demonstrate this. Note that I derived a new ButtonUIElement so I could override the OnClick() as well as make the descendant search easier:

    public class ExplorerBarGroupCloseButtonCreationFilter : Infragistics.Win.IUIElementCreationFilter
    {
        #region IUIElementCreationFilter Members

        public void AfterCreateChildElements(Infragistics.Win.UIElement parent)
        {
            if (parent is UltraExplorerBarGroupHeaderUIElement)
            {
                // find the close button
                CloseGroupButtonUIElement closeButton = parent.GetDescendant(typeof(CloseGroupButtonUIElement)) as CloseGroupButtonUIElement;
                if (closeButton == null)
                {
                    // get the group
                    UltraExplorerBarGroup group = parent.GetContext(typeof(UltraExplorerBarGroup)) as UltraExplorerBarGroup;
                    if (group != null)
                    {
                        // create a new close button
                        closeButton = new CloseGroupButtonUIElement(parent, group);

                        // create the rectangle           
                        // depending on the Style, you may need to adjust the location to aviod the Expansion button.
                        Rectangle parentRect = parent.RectInsideBorders;
                        closeButton.Rect = new Rectangle(parentRect.Right - 40, parentRect.Y, parentRect.Height - 2, parentRect.Height - 2);

                        // add the element to the collection
                        parent.ChildElements.Add(closeButton);

                        // to avoid overlapping the text, you should adjust the text UIElement here.
                        // ...
                    }
                }
            }
        }

        public bool BeforeCreateChildElements(Infragistics.Win.UIElement parent)
        {
            return false;
        }

        #endregion
    }

    public class CloseGroupButtonUIElement : ButtonUIElement
    {
        private UltraExplorerBarGroup group;

        #region Constructor

        public CloseGroupButtonUIElement(UIElement parent, UltraExplorerBarGroup group) : base(parent)
        {
            this.group = group;
            this.Text = "X";
        }

        #endregion //Constructor

        #region Base Class Overrides

        public override UIElementButtonStyle ButtonStyle
        {
            get
            {
                return UIElementButtonStyle.ScenicRibbonButton;
            }
        }

        protected override bool DrawTheme(ref UIElementDrawParams drawParams)
        {
            return false;
        }

        protected override void OnClick()
        {
            if (this.group != null)
                this.group.ParentCollection.Remove(this.group);

            base.OnClick();
        }

        protected override void OnDispose()
        {
            this.group = null;
            base.OnDispose();
        }

        #endregion //Base Class Overrides
    }

Let me know if you have any further questions.

Thanks,

Chris